Flatpak: switch to using the 3.28 runtime again

Gtk was upgraded yesterday to 3.24. This causes a warning about
widgets being drawn without prior allocation. I am not sure yet if
the bug is in the bindings or if gtk's ABI broke.

org.gnome.Podcasts.json

@@ -1,7 +1,7 @@
 {
     "app-id" : "org.gnome.Podcasts",
     "runtime" : "org.gnome.Platform",
-    "runtime-version" : "master",
+    "runtime-version" : "3.28",
     "sdk" : "org.gnome.Sdk",
     "sdk-extensions" : [
         "org.freedesktop.Sdk.Extension.rust-stable"